Computers and a variety of other equipment normally generate digital signals in the form of switched currents or voltages. These signals have either one of two states, usually represented by voltage levels, which are used to indicate a data "1" or "0", respectively. Each such data element is called a binary digit or bit. For short distances, up to a few meters, these digital signals can be carried directly by means of wires from source to destination. However, the electrical characteristics of wires, particularly reactance and resistance, severely limit the rate at which the digital signals may be transmitted over longer distances.
Generally, a modulator is used to convert digital signals into a suitable analog form for long distance communication. A modulator performs this conversion by encoding the digital signals onto a carrier-signal. The modulated carrier-signal occupies a frequency band centred on the carrier-signal. The modulated-carrier signal is then transmitted over a suitable transmission medium. A demodulator acts as the receiver of the modulated carrier-signal and utilizes a demodulation process to extract the modulating signal, or the original data signal. In a bidirectional system, the modulator and demodulator at a station are conveniently combined into a MODEM.
In a communication system which utilizes a MODEM, the primary limitation of the signalling speed will be the bandwidth of the transmission medium. The bandwidth for a typical telephone line is 2700 Hz, extending from 300 Hz to 3000 Hz, and for a radio channel the bandwidth may be 4900 Hz, extending from 100 Hz to 5000 Hz.
An increase in signalling speed within a given bandwidth can be obtained by using schemes that encode two or more bits for each signalling element. For example, in a dibit encoding scheme, combinations of two bits are represented by four different states to be transmitted.
______________________________________ STATE 0 1 2 3 BITS 00 01 10 11 ______________________________________
Each bit combination or state is called a symbol. Encoding two or more bits per symbol leads to an increase in the rate at which bits can be transmitted in a given bandwidth.
A variety of different modulation techniques have been developed to make effective use of the available bandwidth in any given situation. Each modulation technique involves varying one or more of the following three characteristics of a carrier signal: frequency (or, in other words, rate of change of phase angle), phase and amplitude. The following three basic encoding or modulating techniques are used to modulate digital data into an analog form: frequency shift keying (FSK), phase shift keying (PSK) and amplitude shift keying (ASK). Each technique exploits a different characteristic of the carrier-signal in order to convey information in the modulating signal. FSK uses different frequencies to represent different symbols in the modulating signal. The frequency of the modulating signal is varied to represent the information contained in the modulating signal. For convenience in generating symbols and associated timing signals, the frequencies chosen to represent each symbol are made multiples of one another. For example, one fast frequency shift keying (FFSK) technique uses one cycle of 1200 Hz to represent one symbol and one and a half cycles of 1800 Hz to represent another symbol. Each symbol or bit has the same duration. The maximum data rate is limited by the audio bandwidth; therefore, it is advantageous to use the lowest possible audible frequencies.
PSK uses two or more signals of different phase to represent different symbols. Each symbol has the same duration, although the phase of each different symbol differs. For example, in a two symbol system the bits "0" and "1" are distinguished by a 180.degree. phase shift. Similarly, in a four symbol system each symbol is distinguished by means of a 90.degree. phase shift; this is known as quadrature phase shift keying (QPSK).
ASK signals maintain a constant phase and frequency but utilize different amplitudes for each symbol. The carrier amplitude is varied in proportion to the modulating signal. As in each of the other techniques, each of the symbols has the same duration. The signalling rate is limited by the number of bits per symbol, which in turn is limited by the signal to noise ratio of the channel.
In addition to FSK, PSK and ASK, a number of modulation techniques have been developed which employ, either singly or in combination, the above techniques. For example, quadrature amplitude modulation (QAM) uses a combination of amplitude and phase shift keying. However, one feature common to all the modulation techniques discussed above is that all the symbols in the modulated carrier-signal have the same duration.
MODEMS may operate either synchronously or asynchronously. In synchronous operation, a common clock signal is utilized to synchronize the operation of MODEMS at opposite ends of a transmission path, whilst in asynchronous operation, unsynchronized timing references are utilized to demodulate received signals, based on the organization of transmitted data into relatively small frames such that once the beginning of a frame has been recognized, any phase shift of the local clock during decoding of the remainder of the frame will be insignificant; or the local clock can be phase-locked to the incoming signal. In either case, the necessity for a clock signal has been thought to dictate a constant symbol rate.
I have found that, not only is a constant symbol rate not essential to effective MODEM communications, but that the use of a variable symbol rate may have significant advantages under some circumstances.